Evidence is presented which supports the proposal (Erlander, S.R., Enzymologia, 1958, 19, 273-283) that plant glycogen is a required intermediate in the synthesis of starch. Its synthesis temporarily ceases in a specific cell after a period of about three days, and at that point debranching enzymes are then activated which remove its exterior A-chains. The partially debranched glycogen is amylopectin and the removed branches are degraded by soluble starch synthases (SSS I and II) to produce ADPglucose (ADPGlu) which is the sole source of glucose for the production of amylose by the granular bound starch synthase (GBSS). Two independent cytosol/plastid transport systems activate either phosphorylase (from transported ADPGlu) or its back-up system SSS II (from transported Glu-6-Р). Both use ADP glucose pyrophosphorylase in the synthesis of glycogen. The linear chains of debranched amylopectin have the narrow Poisson size distribution, whereas those of linear amylose have the broad size distribution of an A-B condensation polymer. Thus amylose can not be a precursor to amylopectin. Inner, short A-chains, located particularly on the 3rd and 5th tiers of the precursor glycogen, account for changes in the A/B chain ratio and for clusters.